Gamma-Rays from an Asteroid

Perched on the surface of asteroid 433 Eros, NASA's
NEAR spacecraft is beaming back measurements of gamma-rays leaking
from the space rock's dusty soil.

February 27, 2001 -- When NASA's Near Earth Asteroid
Rendezvous (NEAR) spacecraft left for asteroid 433 Eros five
years ago, scientists weren't certain what they would find when
the probe arrived. Was Eros a 30-km fragment from a planet that
broke apart billions of years ago? Or perhaps a jumble of space
boulders barely held together by gravity? Was Eros young or old,
tough or fragile ... no one knew for sure.

But now, after a year in orbit and a daring landing on the
asteroid itself, NEAR Shoemaker is beaming back data that could
confirm what many scientists have lately come to believe: Asteroid
Eros is not a piece of some long-dead planet or a loose collection
of space debris. Instead, it's a relic from the dawn of our solar
system, one of the original building blocks of planets that astronomers
call "planetesimals."

As NEAR Shoemaker was heading for its historic
landing on Feb. 12, 2001, team members hoped the spacecraft
--which was designed to orbit, not land-- would simply survive.
When it did survive, they set their sights a little higher. From
its perch on the surface of the asteroid, NEAR's gamma-ray spectrometer
(GRS)
can detect key chemical signatures of a planetesimal -- data
that scientists are anxious to retrieve.

"The gamma-ray instrument is more sensitive on the ground
than it was in orbit," says Goddard's Jack Trombka, team
leader for the GRS. "And the longer we can accumulate data
the better." NASA recently gave the go-ahead for NEAR's
mission to continue through Feb. 28th, tacking four days onto
an extension granted just after the spacecraft landed.

To do its work the GRS relies partly on cosmic rays,
high-energy particles accelerated by distant supernova explosions.
When cosmic rays hit Eros, they make the asteroid glow, although
it's not a glow you can see with your eyes; the asteroid shines
with gamma-rays.

"Cosmic rays shatter atomic nuclei in the asteroid's
soil," explains Trombka. Neutrons that fly away from the
cosmic ray impact sites hit other atoms in turn. "These
secondary neutrons can excite atomic nuclei (by inelastic scattering)
without breaking them apart." Such excited atoms emit gamma-rays
that the GRS can decipher to reveal which elements are present.

"We can detect cosmic-ray excited oxygen, iron and silicon,
along with the naturally radioactive elements potassium, thorium
and uranium," says Trombka. Measuring the abundances of
these substances is an important test of the planetesimal hypothesis.

Planetesimals
came to be when the solar system was just a swirling interstellar
cloud, slowly collapsing to form the Sun and planets. Dust grains
condensed within that primeval gas. The grains were small, but
by hitting and sticking together they formed pebble-sized objects
that fell into the plane of the rotating nebula. The pebbles
accumulated into boulders, which in turn became larger bodies,
1 to 100 km wide. These were planetesimals -- the fundamental
building blocks of the planets.

For reasons
unknown Eros was never captured by a growing protoplanet. It
remained a planetesimal even as other worlds in the solar system
grew and matured.

Left: Scientist and artist William
K. Hartmann (copyright 2001, all rights reserved) created
this painting of the inner solar system as it may have been less
than 1 million years after the Sun formed. Dust grains are accumulating
into asteroid-sized planetesimals, the building blocks of planets.

Fully-developed planets like Earth are chemically segregated
-- that is, they have heavier elements near their cores and lighter
ones at the surface. Planetary scientists call this "differentiation."
If Eros were a chip from a planet that broke apart, perhaps in
the asteroid belt, it would exhibit chemical signatures corresponding
to some layer from a differentiated world.

For example, Eros might be iron-rich if it came from the core
of such a planet or silicon-rich if it came from the crust.

Instead, "orbital data from the x-ray spectrometer (a
low-energy cousin of the GRS) showed Eros is very much like a
type of undifferentiated meteorite we find on Earth called
ordinary chondrites," says Andrew Cheng, the NEAR project
scientist at Johns Hopkins University Applied Physics Laboratory
(APL), which manages the mission for NASA.

Eros seems to harbor a mixture of elements that you would
only find in a solar system body unaltered by melting (an unavoidable
step in the process of forming rocky planets). But, says Cheng,
there is a possible discrepancy.

"The abundance of the element sulfur on Eros is less than
we would expect from an ordinary chondrite. However, the x-ray
spectra tell us only about the uppermost hundred microns of the
surface, and we do not know if the sulfur depletion occurs only
in a thin surface layer or throughout the bulk of the asteroid."

The
GRS can go deeper, as much as 10 cm below the surface. Although
the instrument can't detect sulfur, it is sensitive to gamma-ray
emissions from other elements such as radioactive potassium that
are indicators of melting. Like sulfur, potassium is a volatile
element -- it easily evaporates when a rock is heated. Finding
plenty of potassium would strengthen the conclusion that Eros
is an unmelted and primitive body.

On the other hand, a widespread dearth of "volatiles"
would hint that Eros isn't so primitive after all.

It might sound like an ivory-tower question, but knowing the
makeup of this asteroid -- both its internal structure and its
chemical composition-- has a practical application. The solar
system is littered with space rocks more or less like Eros, and
many come uncomfortably
close to Earth. One day we may need to blow one apart (or
deflect one without blowing it apart) to avoid an unpleasant
collision. Near-Earth asteroids are also potential mining resources
as humans expand into space. In either case, knowing more about
them is a good idea!

"Our first four data sets are here and they look great,"
says Jack Trombka. "John Goldsten, the lead engineer for
the gamma-ray spectrometer at the Johns Hopkins Applied Physics
Laboratory, has done a fabulous job making the instrument work
on the surface, which is a different
environment than orbit.

"We're just hoping to get as much data as we can before
the mission ends."

NEAR Shoemaker launched on Feb. 17, 1996 - the first in
NASA's Discovery Program of low-cost, scientifically focused
planetary missions -- and became the first spacecraft to orbit
an asteroid on Feb. 14, 2000. The car-sized spacecraft gathered
10 times more data during its orbit than originally planned,
and completed all the mission's science goals before its controlled
descent on February 12, 2001. Funding for the mission extension
comes from the NEAR project.

Asteroids Have Seasons, Too - June 21, 2000 Science@NASA story: Later
this week, the Sun will rise over the south pole of asteroid
Eros, revealing unexplored terrain to the instruments on NASA's
NEAR-Shoemaker spacecraft.

Â More Facts about Eros

Eros
circles the Sun once every 1.76 Earth years. It spins on its
axis once every 5.27 hours.[more]

Eros is about 21 by 8 by 8 miles
(33 by 13 by 13 kilometers) in size. Its shape has been compared
to a shoe, a battered boat, or a peanut. [more]

The gravity on Eros is very weak
but enough to hold a spacecraft. A 100-pound (45-kilogram) object
on Earth would weigh about 1 ounce on Eros. [more]

Eros is a "Near-Earth Asteroid"
or NEA. Its next close approach will come in January 2012, when
it will pass 0.178 AU from our planet. There is no chance of
a collision.[More]

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